In the operation of hotels, manufacturing plants, and other large building complexes, it is customary to provide status sensors such as fire alarms, intrusion detectors, and smoke detectors at sites of interest throughout the complex, and connect them all with a central unit or communications processor for monitoring, recording, or other use. One way to accomplish this is to provide a separate communication line from each sensor to the central unit. It is frequently more efficient to connect the central unit to a small number of remote stations or data gathering panels at strategic locations, as by multiconductor cables, and then extend the connections separately from the stations to individual sensors located nearby. The electric power for the sensors may efficiently be provided by common power supplies located at the remote stations rather than by separate batteries, for example. The signals from the individual sensors are thus collected at remote stations and then transmitted to the central processor.
In order to bring multiple sensor inputs to a central location economically, however, it is more desirable to use a distributed time division multiplexed bus or communication channel that is run throughout a building structure and is common to all of the plurality of widely spaced remote stations or data gathering panels which may provide inputs to the bus.
This type of reporting system is much more economical than the older types of systems which required a separate pair of wires between the central location and each of many remote stations providing inputs to the central location. The labor involved in running a separate pair of wires between each remote station and the central location, even more than the cost of the materials involved, make such "dedicated wire" systems very expensive. By providing a single common communication channel between the central location and all of the remote stations, so that all communication takes place on the same communication channel, labor and materials can both be economized.
Typically each sensor in such a system forms a part of a loop which has a normal status, an alarm status, and a trouble status. Electrically a "normal" status signal is identified by a current within a predetermined range of magnitudes, an "alarm" status signal is identified by a current magnitude greater than the predetermined range, and a "trouble" status signal is identified by a current of magnitude less than the predetermined range.
It is a characteristic of systems of this sort that, while each sensor gives its normal, trouble, or alarm status signal continuously, the signals are transmitted successively and intermittently over the communication channel to the central processor in a repeating sequence. To accomplish this the processor "polls" the remote stations sequentially over the communication channel, thereupon enabling each remote station in turn to return over the communication channel, to the central controller, signals indicative of the various sensor states at that station. It is conventional for each remote station to include means such as an addressed microcomputer, for recognizing when the communication channel is prepared to conduct the signals to the central processor, and means such as a multiplexer for supplying status signals from several sensors to the microcomputer in a repeating sequence.
It has been found that the installation, maintenance, and repair of such systems is rendered difficult due to the fact that there is no ready means whereby servicing personnel working at a particular remote station can determine whether the station is properly in communication with the central processor, or whether a sensor is supplying a normal, trouble, or alarm status signal to the remote station.